This invention relates to a method of forming a coating film of a fluororesin such as, for example, polytetrafluoroethylene on a metallic or nonmetallic substrate surface by using a physical vapor deposition technique.
Fluororesins represented by polytetrafluoroethylene (PTFE) exhibit excellent lubricity and water repellency. Accordingly industrial applications of fluororesins include lubricating and water repelling coating films on various articles. There are many cases where it is desired to form a thin coating film of a fluororesin by a dry coating technique. For example, JP-A 54-20974 shows co-deposition of a metal and a fluororesin to improve lubricity of sliding parts of precision devices such as watches and cameras, and JP-A 55-130133 shows using a fluororesin coating film on a semiconductor chip for enhancement of stability and water resistance of surface areas around electrodes or a protective oxide film surface.
It was proposed to form a fluororesin coating film by plasma polymerization on the surface of a substrate. However, fluoro-monomers suitable for plasma polymerization are very costly, and an intricate apparatus has to be used. Also it was proposed to employ a physical vapor deposition technique such as sputtering or vacuum evaporation for forming a coating film of fluororesin. However, sputtering seems rather disadvantageous because, aside from intricacy of the apparatus, a considerably high discharge voltage is required for effectively bombarding a fluororesin so that the temperature of the substrate rises undesirably. Vacuum evaporation of a fluororesin seems more favorable, but industrial applications of this technique have encountered difficulties attributed to very good thermal stability of fluororesins. For depolymerization and evaporation of a conventional fluororesin it is necessary to heat the fluororesin above 500.degree. C., and such intense heating of the evaporating source places restrictions on the substrate material which should endure the radiant heat from the evaporating source.